Large scale melt predicted at units 2 and 3

26 May 2011

Computer analysis of reactor damage at Fukushima Daiichi has indicated more serious fuel melt has probably occurred than previously thought at units 2 and 3.

Until running computer simulations Tokyo Electric Power Company (Tepco) had estimated core damage based on water level and temperature readings taken at the time. Some of these have been unreliable since the first hours of the accident, probably due to the great strain put upon them by temperature and pressure rises.

It has been found that simulation data and actual readings match fairly closely for the first part of the accident sequence, but significant differences emerge between around the time emergency cooling systems were exhausted and water levels rapidly dropped to the bottom of the fuel assemblies.

This is when seawater injection began, with sensors in the reactor cores showing water levels recovering to about two metres below the top of fuel assemblies. While this would put the cores at risk of damage, a reasonable cooling effect would still be expected thanks to the high thermal conductivity of the zircalloy fuel cladding.

However, the two simulation cases both suggest that water levels did not rise as quickly or by as much.

For unit 2, the first scenario suggests that water only recovered to about three metres below the top of fuel, and the second shows levels below the core entirely. In both cases the fuel temperature is calculated to have shot to around 2700ºC, well beyond the level at which the zircalloy cladding reacts with water to produce hydrogen and more heat. Under these conditions core damage is thought to have begun at 8pm on 14 March, three days and five hours after the tsunami hit.

The first scenario ends with about 54% of the top of the core melting to form a mass supported and surrounded by a number of damaged fuel assemblies. It would reach this steady state on around 18 March, according to the simulation.

The second scenario has only about 12% of the core remaining on the support plate, with the rest having fallen into the water in the the lower part of the reactor pressure vessel. This may have happened by 15 March.

The real situation is likely to be between these cases, but the prognosis compares unfavourably with the estimate of 35% core damage based on readings taken at the time.

Potential end states for units 2 and 3 as simulated by computer codes.On the left, unit 2's scenarios; on the right, unit 3's. Undamaged fuel isindicated by striped lines, damaged fuel is yellow and areas of meltedfuel pellets are orange.

For unit 3 the story is similar. Instrumentation at the plant showed water levels returning to about two metres below the top of fuel, whereas the two simulation cases put levels at about three metres below and just below the core. Temperatures reached damage-inducing levels on 13 March. The two possible core states for unit 3 put forth by the computer also follow the same pattern. The first has 42% of the core melted and merged with a mass of damaged fuel. This sits on a platform of 16% undamaged fuel at the bottom. In the second case, some 94% of the fuel is assumed to have dropped into the water below by the morning of 14 March.

Tepco said the both the second cases indicate the reactor pressure vessels could be damaged, but it considers any such damage to the limited due to temperatures around the vessels shown by a variety of still-functioning sensors. Analysis of unit 1 suggested the entire core had fallen into water in the bottom of the reactor pressure vessel with this somehow allowing water to escape at a limited rate.